Unraveling the Puzzle of the Eclipsing Polar SDSS J015543.40+002807.2 with XMM and Optical Photometry/Spectropolarimetry

The cataclysmic variable SDSS J015543.40+002807.2 is confirmed to be a magnetic system of the AM Herculis type. With an orbital period of 87.13 minutes, it is also the shortest period eclipsing Polar known. Monitoring with XMM-Newton finds a high-state light curve dominated by a single X-ray-emitting accretion pole located slightly prograde of the secondary star. The hard X-ray spectrum is typical of radial shocks on magnetic white dwarfs (kT~10 keV), and there is evidence for a soft X-ray component consistent with reprocessing from the stellar surface. The optical circular polarization is weak (v<~3%) when the accretion rate is high (m_V~15.5), as a result of optically thick cyclotron emission and the apparent competition between two accreting poles. However, in low states (m_V~18), the polarization increases smoothly to the blue, reaching 20% at 4200 Å, and the flux spectrum displays a rich set of thermally broadened cyclotron harmonics that indicate a polar field of 29 MG. The phase interval preceding the 6.5 minute eclipse depicts the development of P Cygni components followed by complete absorption reversals in the emission lines. This phenomenon is not unexpected for a strongly accreting magnetic system viewed through the cool base of the funnel, and high-quality spectroscopy through this interval will likely lead to new insights into the dynamics of magnetic coupling and gas flow onto the white dwarf.

Some of the results presented here were obtained with the MMT Observatory, a facility operated jointly by the University of Arizona and the Smithsonian Institution. Additional results are based on observations made with the Nordic Optical Telescope, operated jointly on the island of La Palma by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.